1. Field of the Invention
The present invention generally relates to a method of and arrangement for measuring capacitances and, more particularly, for measuring low capacitances by alternately switching a reference capacitance and an unkown capacitance to be measured across the input terminals of a measurement oscillator operative for generating an output frequency proportional to the capacitance connected to the oscillator.
2. Description of the Related Art
One starting point for the present invention is the prior art technology disclosed, for example, in Finnish Pat. Nos. 54,664 and 57,319, corresponding to U.S. Pat. Nos. 4,295,090 and 4,295,091, respectively. These patents disclosed a method of and arrangement for measuring low capacitances.
In transducers, capacitive detectors were used for the measurement of various parameters, in particular, pressure, temperature and humidity. The magnitude of the capacitance of the capacitive detectors depended upon the parameter being measured. The capacitances of these detectors were often relatively low, from a few pF to some dozens pF, up to a maximum of about 100 pF. The measurement of low capacitances was erratic, due, for example, to stray capacitances, variations in supply voltage, ambient temperature, and other disturbances. Furthermore, the capacitive detectors were to some extent individual, so that they had unique non-linearity and temperature-dependent characteristics.
In telemeter applications, in particular, when temperature, humidity or pressure, for example, was measured by electric or electromechanical detectors, it was common to connect one or several known reference capacitances to a measurement circuit, the references being highly stable, so that errors of the measurement circuit and/or of the detector could be eliminated.
It was known in the prior art to alternately connect a known reference capacitance and an unknown capacitance to be measured to the input terminals of the measurement circuit. The measurement circuit usually included an RC-oscillator which generated an output frequency. The corresponding output variable of the reference capacitance of the measurement circuit was brought to a correct level by appropriate adjustment of the measurement circuit, or in some other manner.
It was also known in the prior art to use measurement circuits having one reference, in particular, bridge circuits in which the measurement was precise only when the electrical value of the reference was close to the value of the detector, for example, when the bridge circuit was in equilibrium. The greater the difference in value between the detector and the reference, the greater were the various errors, for example, errors caused by changes in the dynamics of the electronic measurement circuit. An advantage of bridge circuits employing one reference was the simplicity of the overall measurement circuit.
An advantage in measurement arrangements with two or more references was accuracy of the measurement, even within wide ranges of measurement. However, the complexity of the measurement method and of the related computation was a serious drawback.